The Extracellular Matrix and Cell Interactions

CHAPTER 11

The Extracellular Matrix and Cell Interactions

Copyright © 2017 John Wiley & Sons, Inc. All rights reserved.


11.1 | Overview of Extracellular Interactions

Materials present outside the plasma membrane play an

important role in the life of a cell.

Most cells in a multicellular plant or animal are organized into

clearly defined tissues in which the component cells maintain a

defined relationship with one another and with the extracellular

materials that lie between the cells.

These interactions regulate such diverse activities as cell

migration, cell growth, and cell differentiation, and they

determine the three- dimensional organization of tissues and

organs that emerges during embryonic development.

An overview of how cells are

organized into tissues and how they

interact with one another and with

their extracellular environment.


11.1 | Overview of Extracellular Interactions

The epidermis has closely

packed cells attached to one

another and to an underlying

non-cellular layer by

specialized contacts.

The dermis is a type of

connective tissue made up

of extracellular material.

Fibroblasts of the dermis

have receptors that mediate

interactions and transmit

messages between

cytoplasmic proteins in the

cell and the environment.

Basal surface of ectodermal

cell, early chick embryoEM: endothelial glycocalyx

in a coronary capillary


11.2 | The Extracellular Matrix

Molecular model

of the glycocalyx

Carbohydrate projections form part of the glycocalyx (or cell coat) on the

outer surface of the plasma membrane.

The glycocalyx mediates cell–cell and cell–substratum interactions,

provides mechanical protection to cells, serves as a barrier to particles

moving toward the plasma membrane, and bind important regulatory

factors that act on the cell surface.



Basement membranes provide mechanical support for the

attached cells, generate signals that maintain cell survival, serve

as a substratum for cell migration, separate adjacent tissues within

an organ, and act as a barrier to the passage of macromolecules.

Basement membranes of capillaries prevent the passage of

proteins out of the blood and into the tissues, particularly important

in kidney function.

Kidney failure in long-term diabetics may result from an abnormal

thickening of the basement membranes surrounding the glomeruli.

Basement membranes also serve as a barrier to invasion of

tissues by cancer cells.

Organized network of

extracellular materials

outside plasma membrane,

provides support and

determines shape and

activity of cell.



The ECM may take diverse forms in different tissues and organisms, yet is

composed of similar macromolecules.

Most proteins inside cells are globular, while those in the ECM are fibrous.

These proteins are secreted into the extracellular space where they are

capable of self-assembling into an interconnected 3D network.

Collagen molecule:

triple helix of three

helical alpha chains

Collagen I molecules

become aligned in

staggered rows


11.3 | Components of the Extracellular Matrix

Collagen

Collagens comprise a family of fibrous

glycoproteins present only in the ECM,

with 28 types found in humans.

Collagen is the single most abundant

protein in the human body constituting

more than 25 percent of all protein.

Collagen is produced by fibroblasts,

smooth muscle and epithelial cells.

Collagen types are restricted to

particular locations, but two or more

different types are often present

together, and mixing of different types

can occur in the same fiber.

EM of human

collagen fibrils after

metal shadowing

Atomic force micrograph

of a collagen fibril surface



Collagen

All collagen molecules are trimers

consisting of three polypeptide chains,

called α chains.

Along part of their length, the three

polypeptide chains are wound around

each other to form a rod-like triple helix.

Collagen molecules contain large

amounts of proline, and many of the

proline/lysine residues are hydroxylated

to form hydrogen bonds between chains.

Ascorbic acid is required as a coenzyme

by the enzymes that add -OH groups to

the lysine and proline residues.

EM of human

collagen fibrils after

metal shadowing

Atomic force micrograph

of a collagen fibril surface



Collagen

Type I, II, and III collagens are

fibrillar because they assemble into

cable-like fibrils and then are

packaged into thicker fibers.

The fibrils are strengthened by

covalent cross-links between lysine

and hydroxylysine residues on

adjacent molecules.

This cross-linking process continues

through life and may contribute to

the decreased skin elasticity and

increased bone brittleness among

the elderly.

Corneal stroma: layers of collagen

fibrils of uniform diameter and

spacing arranged at right angles



Collagen

Collagen provides the insoluble

framework that determines many of

the mechanical properties of the

matrix.

Tissue properties can often be

correlated with the 3D organization of

its collagen (e.g. tendons, cornea).

In the cornea, the fibrils of each layer

are parallel to other fibrils in the layer

but perpendicular to the fibrils in the

layer on either side. This structure

provides strength and promotes the

tissue’s transparency.

Proteoglycans – protein-

polysaccharide complex, with

a core protein attached to

glycosaminoglycans (GAGs).

Have a repeating disaccharide

structure, -A-B-A-B-, where A

and B represent two different

sugars.

Proteoglycans of the ECM

may be assembled into

gigantic complexes by linkage

of their core proteins to a

molecule of hyaluronic acid, a

non-sulfated GAG.

Schematic representations

of a single proteoglycan,

repeating disaccharide

structure of GAGs, and

linkage to hyaluronic acid

to form a giant complex



Proteoglycans

Electron micrograph of a

proteoglycan complex isolated

from cartilage matrix.



Proteoglycans

Negative charges on the sulfated

GAGs, attract cations, which in turn bind

large numbers of water molecules.

As a result, proteoglycans form a

porous, hydrated gel that fills the ECM

to resist crushing (compression) forces.

Collagens and proteoglycans give

cartilage and other ECM strength and

resistance to deformation.

Human fibronectin molecule

consists of two similar polypeptides

joined by disulfide bonds.



Fibronectin

Fibronectin consists of a linear array

of 30 Fn domains to give a modular

construction, which combine to form

five or six larger functional units.

Fn-type domains are found in blood

clotting factors and receptors.

Each of the two polypeptide chains

that make up fibronectin contain:

1) Binding sites for ECM molecules

such as collagens, proteoglycans,

and other fibronectin molecules.

2) Binding sites for receptors on the

cell surface.

Primordial germ cells (green)

migrating along a tract of laminin (red)

from the dorsal mesentery to the

developing gonad.



Laminin

Laminins comprise a family of at least

15 extracellular glycoproteins.

Laminin has three polypeptide chains

linked by disulfide bonds and

organized into a molecule resembling

a cross with three short arms and one

long arm.

They can greatly influence a cell’s

potential for migration, growth, and

differentiation.

Primordial germ cells possess a cell

surface protein that adheres strongly

to a subunit of laminin.

A model of the basement membrane

scaffold. Basement membranes contain

two network-forming molecules, collagen IV

(pink), and laminin (green) that are

connected by entactin molecules (purple).



Laminin

Laminins can bind to other laminin

molecules, proteoglycans, and

other components of basement

membranes.

Laminin and type IV collagen

molecules of basement

membranes may form separate,

interconnected networks.

These interwoven networks give

basement membranes both

strength and flexibility.


11.4 | Dynamic Properties of the Extracellular Matrix

Although images can portray the ECM as a static structure, in

actuality it can exhibit dynamic properties both in space and time.

Spatially, ECM fibrils can stretch several times their normal

length as they are pulled on by cells and contract when tension is

relieved.

Temporally, ECM components are subject to continual

degradation and reconstruction, even the calcified matrix of

bones.

These processes serve to renew the matrix and to allow it to be

remodeled during embryonic development or following tissue

injury.


11.4 | Dynamic Properties of the Extracellular Matrix

ECM degradation is accomplished by a family of zinc-containing

enzymes called matrix metalloproteinases (MMPs) secreted into

the extracellular space or anchored to the plasma membrane.

As a group, MMPs can digest nearly all of the diverse ECM

components, although individual family members are limited as

to the types of extracellular molecules they can attack.

The physiological roles of MMPs are thought to be involved in

tissue remodeling, embryonic cell migration, wound healing, and

the formation of blood vessels.

Inappropriate MMP activity has been implicated in arthritis, tooth

and gum disease, formation of blood clots and heart attack, and

tumor progression.


11.5 | Integrins

Integrins are a family of membrane proteins found only in animals

that play a key role in integrating the extracellular and intracellular

environments, and are important for the attachment of cells to their

extracellular microenvironment.

On the outer side of the plasma membrane, integrins bind to a

diverse array of ligands present in the extracellular environment.

On the intracellular side of the membrane, integrins interact either

directly or indirectly with dozens of different proteins to influence the

course of events within the cell.

Integrins are composed of two membrane-spanning polypeptide

chains, an α chain and a β chain, that are non-covalently linked.

Eighteen different α subunits and eight different β subunits have

been identified.


11.5 | Integrins

Only about two dozen different

integrins have been identified on

the surfaces of cells, each with a

specific distribution within the body.

Whereas many subunits occur in

only a single integrin heterodimer,

the β1 subunit is found in 12

different integrins.

Most cells possess a variety of

different integrins, and conversely,

most integrins are present on a

variety of different cell types.

Many ECM proteins through an

arginine-glycine-aspartic (RGD)

motif.

EMs and ribbon drawings of the extracellular domains of an integrin

(avb3) in the “bent”/inactive and “upright”/active conformation.

Changes driven by divalent metal ions.


11.5 | Integrins

The bent conformation of integrin

corresponds to its inactive state,

incapable of binding a ligand.

Integrins with a bound ligand are

found in an upright conformation.

The transmembrane domains of the

two subunits are in close proximity,

held together by noncovalent

interactions between residues of

the two helices.

The cytoplasmic domains bind a

wide array of proteins including

talin, which causes separation of

the α and β subunits via “inside-out”

signaling.


11.5 | Integrins

Integrins have two major activities: adhesion of cells to their

substratum (or to other cells) and transmission of signals between

the external environment and the cell interior.

Integrin-ligand binding can induce a conformational change at the

cytoplasmic end of the integrin, especially its β subunit.

Outside-in signals can induce a conformational change in talin,

initiating a cascade of events leading to the polymerization of actin

filaments.

Cytoplasmic protein kinases (e.g. FAK and Src) can be activated to

phosphorylate other proteins; this chain reaction could lead

activation of a specific group of genes.


11.5 | Integrins

Outside-in signals transmitted by integrins can influence

differentiation, motility, growth, and cell survival.

Most malignant cells are capable of growing in liquid

suspension, their survival no longer depends on integrin binding.

Normal cells can only grow and divide if they are cultured on a

solid substratum to transmit life-saving signals to the interior of

the cell; if they are placed in suspension cultures, they die.


11.6 | Anchoring Cells to Their Substratum

Steps in the process of cell spreading.

Since it is easier to study the cell

interactions with a culture dish than

with an ECM inside an animal, in

vitro experiments have been

instrumental in understanding cell-

matrix interactions.

At first, the cell has a rounded

morphology, but once the cell makes

contact with the substratum, it sends

out projections that form increasingly

stable attachments.

Over time, the cell flattens and

spreads itself out on the substratum.

Cultured cell: actin filaments

(gray-green), integrins (red);

sites of focal adhesions

Amphibian cell processed for

quick-freeze, deep-etch analysis



Focal Adhesions

Cultured cells are anchored to the

surface of the dish only at scattered,

discrete sites, called focal

adhesions.

Focal adhesions play a key role in

cell locomotion, during which the

integrins develop transient

interactions with extracellular

materials.

Focal adhesions are dynamic

structures that can be rapidly

disassembled if the adherent cell is

stimulated to move or enter mitosis.

Focal adhesions are sites where

cells adhere to their substratum and

send signals to the cell interior



Focal Adhesions

Integrin cytoplasmic domains are

connected to cytoskeletal actin

filaments through stratified layers

of adaptor proteins (e.g., talin, α-

actinin and vinculin).

Pulling of the ECM can lead to

activation of protein kinases, such

as FAK or Src, to dramatically alter

cell behavior.

Hemidesmosomes: EM

and schematic diagram.



Hemidesmosomes

Cell-matrix attachment in vivo is seen

at the basal surface of epithelial cells,

anchored to the underlying basement

membrane by a specialized adhesive

structure called a hemidesmosome.

Hemidesmosomes contain a dense

cytoplasmic plaque with keratin

filaments coursing outward into the

cytoplasm.

Keratin filaments are linked to the ECM

by integrins, which can transmit signals

to alter cell shape and activities.

Hemidesmosomes: EM

and schematic diagram.



Hemidesmosomes

In the autoimmune disease bullous pemphigoid, antibodies are made

towards proteins present in these

adhesive structures.

These auto-antibodies cause the

epidermal layer to lose attachment to

the underlying basement membrane

and result in severe skin blistering.

Epidermolysis bullosa results from

genetic alterations in hemidesmosomal

proteins, including the α6 or β4 integrin

subunit, collagen VII, or laminin-5.

Ectoderm and mesoderm from an

early amphibian embryo re-associate

after initial random dissociation


11.7 | Interactions of Cells with Other Cells

Organs have a complex architecture

involving a variety of different cell

types, although the mechanisms

behind this organization are not fully

understood.

This process may depend on

selective interactions between cells

of the same type, as well as

between cells of a different type.

Dissociation-reaggregation

experiments showed that a

suspension of single cells would

initially aggregate to form a mixed

clump.

Light micrograph: mixed pre-cartilage

cells from a chick limb and chick heart

ventricle cells re-sort based on cell type



Over time, cells would redistribute

themselves so that each cell adhered

only to cells of the same type.

Once segregated, cells would

differentiate into many of the

structures they would have formed

within an intact embryo.

Four distinct families of integral

membrane proteins play a major role

in mediating cell–cell adhesion: (1)

selectins (2) certain members of the

immunoglobulin superfamily (IgSF),

(3) certain members of the integrin

family, and (4) cadherins.

Schematic of the three

selectins and their ligand



Selectins

Selectins are a family of membrane

glycoproteins that bind to specific

oligosaccharides on the surfaces of

neighboring cells.

“Lectin,” is a term for a compound that

binds to specific carbohydrate groups.

Selectins have a small cytoplasmic

segment, a single membrane-spanning

domain, and a large extracellular portion.

Three cell-specific selectin types:

E-selectin, endothelial cells;

P-selectin, platelets and endothelial cells;

L-selectin, leukocytes (white blood cells).

Schematic of the three

selectins and their ligand



Selectins

Binding of selectins to their

carbohydrate ligands requires calcium.

As a group, selectins mediate transient

interactions between circulating

leukocytes and vessel walls at sites of

inflammation and clotting.

Cell–cell adhesion from homotypic

interactions of two L1 molecules

through Ig domains at the N-termini.



The Immunoglobulin Superfamily

Antibodies are immunoglobulin

proteins with Ig domains, 70-110

amino acids organized into a tightly

folded structure.

The human genome encodes 765

distinct Ig domains, making it the

most abundant domain in human

proteins.

These proteins are members of the

immunoglobulin superfamily, or IgSF,

and most are involved with mediating

interactions of lymphocytes with cells

required for an immune response

(e.g., macrophages, lymphocytes,

and target cells).

Cell–cell adhesion from homotypic

interactions of two L1 molecules

through Ig domains at the N-termini.



The Immunoglobulin Superfamily

VCAM (vascular cell-adhesion

molecule), NCAM (neural cell-

adhesion molecule), and L1

(also called L1CAM), mediate

adhesion between nonimmune

cells.

NCAM and L1 have roles in

nervous system development.

Various types of proteins serve

as ligands for IgSF cell surface

molecules, such as integrins.

Schematic of two adhering cells due to

interactions between cadherins projecting

from the plasma membrane of each cell



Cadherins

Cadherins are family of

glycoproteins that mediate Ca2+-

dependent cell–cell adhesion and

transmit signals from the ECM to

the cytoplasm.

Cadherins typically join cells of

similar type to one another and do

so predominantly by binding to the

same cadherin present on the

surface of the neighboring cell.

Cadherins may be the single most

important factor in molding cells into

cohesive tissues in the embryo and

holding them together in the adult.

Diagram and EM showing the junctional

complexes on the lateral surfaces of a

simple columnar epithelial cell


11.8 | Adherens Junctions and Desmosomes

Cadherins are found in specialized

intercellular junctions: adherens

junctions (AJs) and desmosomes.

AJs are commonly found in

epithelia as “belts” (zonulae adherens) that encircle cells near

their apical surface, binding a cell

to its surrounding neighbors.

These calcium-dependent linkages

formed between the extracellular

domains of cadherin molecules that

bridge the 30–nm gap between

neighboring cells.

Cadherins link the two cells across

a narrow extracellular gap



The cytoplasmic domain of

cadherins is linked by α- and β-

catenins to cytoplasmic proteins

including actin filaments.

Similar to integrins of a focal

adhesion, the cadherin clusters of

an adherens junction:

(1)Connect the external

environment to the actin

cytoskeleton and

(2)Provide a pathway for signals to

be transmitted from the exterior to

the interior

Adherens junctions situated that

line the walls of blood vessels

transmit signals that ensure the

survival of the cells.

Model: molecular architecture

of a desmosome



Desmosomes (or maculae adherens)

are disk-shaped adhesive junctions

found in a variety of tissues subjected

to mechanical stress, such as cardiac

muscle, epithelial layers of the skin

and uterine cervix.

Desmosomes contain cadherins with

a different domain structure and

are referred to as desmogleins and

desmocollins.

Dense cytoplasmic plaques serve as

sites of anchorage for looping

intermediate filaments.

EM: apical

region of

adjoining

epithelial

cells


11.10 | Tight Junctions: Sealing the Extracellular Space

Tight junctions (zonulae occludens),

occur between neighboring epithelial

cells, and are located at the very

apical end of the junctional complex

between adjacent cells.

EM reveals that the adjoining

membranes make contact at

intermittent points, rather than being

fused over a large surface area.

The points of cell–cell contact are

sites where integral proteins of two

adjacent membranes meet within

the extracellular space.

Tight junction model

showing intermittent

contact points

between proteins

from two apposing

membranes

Electron micrograph of a section through

a gap junction perpendicular to the plane

of the two adjacent membranes


11.11 | Gap Junctions and Plasmodesmata: Mediating

Intercellular Communication

Gap Junctions

Plasma membranes of a gap junction

contain channels that connect the

cytoplasm of one cell with the

cytoplasm of the adjoining cell.

The passage of ionic currents through

gap junctions plays a pivotal role in

numerous physiologic processes.

Gap junctions are sites between

animal cells that are specialized for

intercellular communication.

Gap junctions come very close to one

another but do not make direct

contact.

Schematic model of a gap

junction showing the

arrangement of six connexin

subunits to form a connexon


11.11 | Gap Junctions and Plasmodesmata: Mediating

Intercellular Communication

Gap Junctions

Gap junctions are molecular

“pipelines” that pass through the

adjoining plasma membranes and open

into the cytoplasm of the adjoining

cells.

They are composed of an integral

membrane protein connexin, and

organized into multisubunit complexes,

connexons, that span the membrane.

A connexon is composed of six

connexin subunits arranged in a ring

around a central opening, or annulus,

around 1.5 nm in diameter at its

extracellular surface.

The Extracellular Matrix and Cell Interactions

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